Electrodeposition facilities, in particular facilities intended for the electrolytic extraction of non-ferrous metals, typically use at least one electrolysis cell comprising a plurality of unit cells each of which comprises an anode and a cathode, generally located in the electrolysis bath in an alternating and mutually parallel position.
In the case of facilities for the electrolytic extraction of non-ferrous metals such as copper, cobalt, zinc or nickel, the metal is deposited as the electrical current passes through the cathode of each unit cell and the metal is collected at periodical intervals by removing the cathodes from their seats. In the situations described above deposition of the metal may take place non-uniformly and give rise to dendritic formations, that is localised deposits which grow towards the opposite anode at an increasing rate with the passage of electrical current, ultimately coming into direct electrical contact with the latter. In this case the short circuit produced between the electrodes can draw current off from the other electrolysis cells, reducing the quality and quantity of the metal produced, and give rise to a local increase in the anode temperature which can cause it to be damaged. In modern anodes made of grids or stretched sheets of titanium, or other valve metal, these undesirable effects can give rise to extensive irreversible damage.
In general damage to the anodes involves greater maintenance costs for the plant, a lesser quantity of metal produced and possible further damage associated with forced shutdown of the system.
It has been observed that in typical facilities for the electrolytic extraction of non-ferrous metals short circuits caused by dendrites are typically concentrated during the period of time corresponding to the last 25-30% of the length of each collection cycle, depending upon the operating conditions in the facility. In medium-sized electrolytic facilities for the extraction of copper operating at a current density of approximately 400-460 A, for example, short circuits caused by dendrites typically occur during the last 20-24 hours of each cycle of average length 4-5 days.
The use of an anode enclosure comprising a permeable material, for example a porous separator of polymer material or an ion-conducting membrane, as described in application WO2013060786, is ineffective in blocking or slowing the growth of dendrites for sufficient time to reduce the number of actions which have to be taken by operators in the event of electrical contact and limit their urgency.
The inventors have observed that the use of a protective screen of conductive material placed so as to protect the anode can slow down the growth of dendrites for an average period of approximately 8-10 hours, but if there is contact with the dendritic formation damage to the anode is generally non-negligible because of high current transport through the conducting screen. Furthermore, on contact with the dendrite, the conductive screen reaches the cathode potential and tends to be coated with metal. The inventors have observed that metal deposited on the screen is not fully dissolved when the cell is restarted after collection operations, but on the side facing the anode can detach as fragments, which may even be large, that are capable of causing further short circuits with the anode when the plant is restarted, damaging it as a consequence.
The need for a system capable of blocking or in any event delaying the growth of dendritic formations in the direction of the opposite electrode for a sufficient number of hours to minimise the number and urgency of actions by the personnel operating a facility has therefore been reviewed. In particular it is felt that during night-time shifts it may happen that operators are not present in sufficient numbers to ensure that action is taken in good time in the event of a short circuit between electrodes. In addition to this, the facility may not be provided with cell current monitoring systems capable of indicating the presence of abnormalities in current distribution quickly and accurately. A system capable of retarding the growth of dendritic formations for a period of at least 12 hours, preferably at least 18-24 hours, is therefore desirable.
It is also desirable that whenever a short-circuit situation should become established between the electrodes of a unit cell through contact via a dendritic formation, the damage caused to the electrodes will be such as to keep the electrode in question functioning and not have an adverse effect on the quantity and quality of production, thus helping to reduce maintenance costs for the facility.